The innate immune responses of a host are the first line of defense against infectious disease. The principal challenge for the host is to detect the pathogen and mount a rapid defensive response. A group of proteins that comprise the Toll-like Receptor (TLR) family performs this role in vertebrate organisms. The TLRs are key proteins that allow mammals, whether immunologically naïve or experienced, to detect microbes. For instance, TLR5 recognizes bacterial flagellin, the major structural protein of flagella. Flagellin is a 55-kD protein monomer obtained from bacterial flagella, polymeric rod-like appendages that extend from the outer membrane of Gram-negative bacteria. Gram negative flagellin is a potent inducer of innate immune effectors, such as cytokines and nitric oxide. Vaccines that induce mucosal immunity are likely to provide the most effective protection against pathogens, such as influenza and other viruses.
Since the outbreak of a highly pathogenic avian influenza virus (HPAI) H5N1 variant in 1997 in Hong Kong, there have been increased concerns about the threat of a new pandemic that may cause widespread fatal infection in humans. Although the transmission of avian influenza viruses from birds to humans is a rare event, both the continuing increase of infected human cases and the high mortality rates suggest the persisting threat of an H5N1 pandemic. There is evidence that the 1918 pandemic virus, which caused an estimated 40 million deaths, was an avian virus directly adapted to humans. Although two classes of antiviral drugs targeting the viral matrix protein M2 and neuraminidase, respectively, are available against influenza A viruses, financial and supply limitations as well as frequent drug resistance may limit the ability to utilize these drugs for preventing a new pandemic. It is well recognized that an effective vaccine is the primary strategy for protection against an emerging pandemic.
Currently, an inactivated influenza vaccine is the dominant form used, although a live attenuated (cold-adapted) influenza virus vaccine has also been introduced. However, the emergence of a new pandemic strain could easily overwhelm the present capacity of vaccine production, which is based on embryonic hens' eggs. There are additional concerns that biosafety containment facilities may be needed for virus-based vaccine production, and a period of 6 to 9 months would be required. A safe, convenient, and more reliable alternative is needed as a countermeasure to the emerging challenge.
As a new form of vaccine candidate, virus-like particles (VLPs) have been reported to be potent vaccines for a variety of pathogenic viruses (Koutsky et al., (2002) N. Engl. J. Med. 347:1645-1651; Leclerc et al., (2007) J. Virol. 81:1319-1326; Revaz et al., (2007) Antivir. Res. 76:75-85; Skountzou et al., (2007) J. Virol. 81:1083-1094; Zhang et al., (2007) Scand. J. Immunol. 65:320-3). VLPs elicit immune responses including both B-cell-mediated antibody and specific T-cell-mediated cellular responses to protect experimental animals against lethal influenza virus challenge (Bright et al., (2007) Vaccine 25:3871-3878; Galarza et al., (2005) Viral Immunol. 18:244-251; Matassov et al., (2007) Viral Immunol. 20:441-452; Pushko et al., (2005) Vaccine 23:5751-5759; Quan et al., (2007) J. Virol. 81:3514-3524). However, though VLPs provide an attractive platform for designing vaccines against a possible new influenza virus pandemic strain, they resemble the current vaccines in inducing immune responses that are predominantly subtype specific. An important advance would be the development of new vaccines with enhanced breadth of immunity, which could potentially be used to prevent infection by newly emerging variants, including influenza viruses of other subtypes.